Individuals who require vision correction are presented with a choice between wearing glasses, i.e. corrective lenses mounted in frames, or contact lenses placed directly in intimate contact with the cornea on the surface of the eye. While glasses are successful in correcting many vision problems, they are easily damaged, require repeated cleaning, tend to change the appearance of the wearer and are somewhat heavy and uncomfortable. Contact lenses have minimal mass and size, are easy to use, do not substantially change the user's appearance, and can correct vision problems without reducing field of vision.
Both hard and soft contact lenses are available. Hard lenses are rigid or semi-rigid and are commonly made from methyl methacrylate polymers which are optically excellent and easily machined and molded. Soft lenses are made from hydrophilic polymeric compositions, for example a hydroxyalkylmethacrylate polymer. After soaking in an aqueous medium, soft lenses are flexible and readily conform to the shape and contour of the corneal surface of the eye. Soft contact lenses are somewhat more expensive than hard lenses, and require somewhat more care.
The corneal surface of the eye comprises epithelial cells that respire by obtaining oxygen by diffusion from tear fluid and by diffusing carbon dioxide from the cell into the tear fluid. Oxygen is required by the cells in energy producing metabolism and carbon dioxide is a major waste product of metabolism. Many contact lens materials are substantially impermeable to the exchange of oxygen and carbon dioxide between tear fluid and the corneal epithelial cells. Further, the intimate contact and position of the lens substantially prevents tear fluid from circulating over the epithelial cells covered by the lens. As a result corneal epithelial cells covered by a contact lens can become starved for oxygen and the concentration of carbon dioxide can build up inside the cell, causing corneal edema and often extreme discomfort. In the absence of means to provide oxygen and carbon dioxide exchange between the corneal epithelium and tear fluid, the user must restrict the use of the lenses to comparatively short periods.
In the past many design modifications and many different materials have been tried to increase flow of tear fluid and reduce oxygen starvation. Lenses have been designed with small channels under the lens which permitted the flow of fluid. The selection of compositions for contact lens manufacture should be made with the following characteristics in mind. The lenses must be optically clear, easily machined and molded, highly permeable to the exchange of oxygen and carbon dioxide between tear fluid and the corneal epithelium, dimensionally and mechanically stable, water wettable to avoid eye irritation, and biochemically inert to avoid eye inflammation.
Soft lenses, made from hydroxy alkyl methacrylate polymers, that permit sufficient oxygen and carbon dioxide exchange, tend to be fragile, can be easily contaminated by bacteria, and are of a size that is too large for users who prefer or require ultrathin lenses.
Recently optically clear lenses, permeable to exchange of oxygen and carbon dioxide, have been made from polymers comprising styrene or alkyl substituted styrene wherein the alkyl group can be on the vinyl portion of the styrene molecule or on the aromatic portion. See U.S. Pat. Nos. 4,243,790; 4,228,269 and U.K. Pat. Nos. 1,395,501; 1,436,705; and 1,478,455. We have found that these lens materials tend to be hydrophobic and can cause eye and eyelid irritation since tears cannot wet the lens. A lens which cannot be wetted tends to be somewhat abrasive to the eye and lid, causing wearer discomfort. Hydrophilic monomers can be added during polymerization of the lens material, however well known hydrophilic monomers such as N-vinyl pyrollidone and others can reduce the mechanical and dimensional stability of the lens.
Accordingly, a substantial need exists for a material for hard contact lenses which is optically clear, dimensionally stable, permeable to oxygen and carbon dioxide exchange between the corneal epithelium and tears, wettable by tears, and substantially non-abrasive and non-irritating to the eye and lid.
We have found a contact lens material, which combines the properties of oxygen and carbon dioxide exchange permeability, wettability, dimensional and mechanical stability with optical clarity, which comprises:
(a) styrene, a substituted styrene compound or mixtures thereof,
(b) an ethylenically polyunsaturated crosslinking compound,
(c) an ethylenically unsaturated silicone containing compound, and
(d) an .alpha.,.beta.-ethylenically unsaturated dicarboxylic acid anhydride compound.